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C7-6207b
C7-6207b (fully designated EPIC 216876207b) is a candidate Super-Earth, possibly an ocean planet, orbiting a red dwarf in K2 Campaign 7. It resides well within the conservative habitable zone of the system and is about 80% bigger than Earth. First spotted on August 30th, 2017 by ProtoJeb21, C7-6207b was noted as an extremely likely extrasolar object taking about a month to orbit a red dwarf about 1/4 the size of the Sun. However, later analyses in early February 2018 revealed that the star could be much smaller, and the planet instead an icy ringed Super-Earth. History and Discovery C7-6207b was found by Exoplanet Explorers user ProtoJeb21, who first reported the candidate on August 30th, 2017. Initial estimates put the planet at about 1.53 Earth radii with a year of 32.343 days, within the outer part of the system's habitable zone. It was originally predicted to be a large, somewhat chilly terrestrial planet. A later analysis refined the radius of C7-6207b at about 1.84 R_Earth, which would likely make it an oceanic planet. On February 1st, 2018, the planet candidate was analyzed yet again using a better processed set of data. C7-6207b was found to be about 1.775 R_Earth in a 32.343611-day orbit. However, two major problems remained. The parameters for the host star, EPIC 216876207, were highly uncertain and meant that it could be as large as over 2.5 solar radii. In addition, C7-6207b had transits lasting over 6.5 hours, far too long for a planet orbiting a red dwarf and leading weight to the hypothesis that it is an eclipsing binary. Further analysis revealed two likely solutions to these problems. It was revealed that data gathered by the 2MASS program indicated that EPIC 216876207 was much smaller than expected, likely close in size to Kepler-42 (0.17 solar radii, 0.13 solar masses). This would make C7-6207b much smaller and colder at ~1.23 R_Earth and ~178 K (-139 F). The long transit durations were theorized to be a result of a large planetary ring, which was found to be stable around a planet like C7-6207b. The possibility that the candidate is a so-called blended eclipsing binary still remains. Characteristics Radius and Mass C7-6207b was only detected with the transit method, meaning that just its radius is known. The dip in starlight caused by the candidate passing in front of its star relative to Earth revealed a planetary radius of about 1.775 Earth radii (77.5% bigger than Earth). While a mass is not known, given its radius and the fact that above 1.6 R_Earth planets tend to have high amounts of water, C7-6207b is predicted to be about 5.5 times as massive as Earth. This would give it a density of 5.42 g/cm^3 and a surface gravity 1.75 times that of Earth. The likely smaller radius of its host star means that C7-6207b is more likely to be around 1.23 R_Earth, with a presumed mass of about 2.25 M_Earth. Its density would be 6.67 g/cm^3 and its surface gravity would be 1.49g. Temperature Prior to the re-analysis of its star, C7-6207b was presumed to be within the habitable zone of its star. Its calculated equilibrium temperature, given an Earth-like albedo of 0.3, would be about 220 Kelvin (-64 Fahrenheit; -53 Celsius). While this is far below the freezing point of water, a thick atmosphere would be enough to warm at least parts of the planet up to above 273 K (32 F; 0 C). It was calculated to receive 55% the sunlight that Earth gets. However, given that the host star is likely smaller and cooler, so is C7-6207b. Its new equilibrium temperature is about 178 K (-139 F; -95 C), and it is expected to get about 23% the sunlight Earth gets. No matter how much atmosphere C7-6207b would have, it would be too distant and too cold to be warmed up to temperatures where liquid water could pool on the surface. Orbit C7-6207b takes 32.343611 Earth days to orbit its host star at a distance of about 0.1233 AU. For comparison, Mercury takes about 88 days to orbit the Sun at a distance of 0.38 AU, or 38% the distance between the Sun and Earth. An orbit like this around our Sun would result in a scorched world, but C7-6207 orbits a much dimmer star. Host Star The parent sun of C7-6207b is EPIC 216876207. It has a very poorly constrained radius, mass, and temperature, but it is most likely a red dwarf. The best-fit parameters provided by Huber et al give EPIC 216876207 a radius of 0.245 R_Sun, a mass of 0.239 M_Sun, and an effective temperature of 3534 K. This would give it about 0.84% the luminosity of the Sun. However, the parameters for the star are more likely to be around those of the tiny dwarf star Kepler-42 (0.17 R_Sun, 0.13 M_Sun, 3068 K, 0.0024 L_Sun). This was determined using EPIC 216876207's J-mag, H-mag, and K-mag values provided on the database ExoFOP. Subtracting H-mag from J-mag and K-mag from H-mag resulted in values of 0.486 and 0.25. For comparison, Kepler-42 has nearly identical values of 0.492 and 0.22. Potential Rings On February 1st, 2018, it was proposed that the abnormally long transits of C7-6207b could be caused by a planetary ring system. Not only could rings be large enough to create the anomalies, but they are likely stable, given that a stable orbit for satellite objects around the planet are well between the Hills Sphere and Roche Limit of C7-6207b. This holds true for both size models of the system. For the smaller size model, the rings would need to be about 6.28 times as wide as Earth, or about 80,020 km wide. For comparison, Saturn's rings are about 282,000 km across. For Huber's parameters for the system, the rings would need to be about 5.34 times as wide as Earth (~68,042 km).Category:Habitable Zone Planets Category:Super-Earths Category:Ocean World Candidates